RU2174465C2 - Thermoplastic resin blank - Google Patents

Abstract

FIELD: production of containers. SUBSTANCE: blank is designed for industrial production of thermoplastic resin containers to be filled with liquids of high temperature and/or saturated liquids. Proposed blank has upper or neck section, middle section and end section. Middle section includes lower subsection at its lower side near end section. Lower subsection is intended for stretching at blow moulding with inclusion of support zones of bottle and adjacent zones. Angle of inclination of inner wall of blank and outer wall of above-indicated lower subsection relative to blank axis is 5-10 deg. EFFECT: provision of thermally stable reusable container with shape of bottom such as that of champagne bottle bottom. 6 cl, 8 dwg

Description

The invention relates to a workpiece and a corresponding manufacturing method adapted for the manufacture on an industrial scale of containers of thermoplastic resin, in particular of polyethylene terephthalate (PET) and polypropylene (PP), intended to be filled with liquids that may have a high temperature and / or be saturated, t .e. incorporate gaseous CO ₂ (carbon dioxide).

 In the field of technology encompassing technology and installations for the manufacture of such containers, there are a number of developments and improvements aimed at obtaining manufacturing methods and related plants capable of manufacturing such containers with increased reliability, at the lowest cost, in a universal way, with an increase in the level of quality in a highly competitive industrial the context of very large-scale production.

 It is well known that the aforementioned manufacturing methods according to typological features can be schematically divided into two main groups: one-stage and two-stage methods.

 One-step methods are called because with their help it is possible to obtain the so-called preformed billet and transfer the said billet from the injection mold or extrusion head (after cooling it to the appropriate temperature) to the conditioning station, where the billet is maintained until the temperature is uniformly distributed. molecular orientation. After this, the previously formed preform is moved into a blow mold in which it is finally given the necessary shape.

 A characteristic feature of any one-step method is the uneven distribution of heat over the cross-sectional area of the wall thickness of the workpiece when the latter is moved from a die or an extrusion die. Various methods have been patented regarding changes in the processing time and temperature of the workpiece at the time it is removed from the injection mold, with the goal of optimizing the production cycle time.

 In the patent literature relating to one-step processes, in all cases, the final molding of a thermoplastic resin container is disclosed, which to some extent makes it possible, through the conditioning station, to achieve a uniform wall temperature over their cross-sectional area, said temperature corresponding to the preferred molecular orientation temperature pitches.

 Two-stage methods are called because a bottle made by blow molding is obtained at two different stages, which can be carried out even after very significant periods of time have elapsed between them. In fact, the real advantage of this technology lies in the fact that the whole process is divided into two stages, which under normal conditions are carried out with a significant gap between them both in time and place, which provides increased flexibility from a technical, production and marketing point of view .

 Individual preforms are made in the first step of the aforementioned method, after which these preforms are usually either stored in situ or transferred to the production premises of the end user or processor.

 In the second step of the aforementioned method, said blanks are reconditioned to the required temperature and immediately thereafter are blow molded into the required final products, i.e. bottles.

 Along with increased flexibility, the two-stage methods also potentially provide significant possible savings due to an increase in the scale of production, since the same manufacturer can produce blanks at the same installation, which can subsequently be used to make bottles of various types.

 However, the two-stage methods have a significant inherent disadvantage, the essence of which is the increased energy consumption due to the fact that at the second stage or stage of the workpiece must be completely reconditioned, i.e., heated to the optimum temperature necessary for the subsequent implementation of the operation of blow molding.

In both of the aforementioned one- and two-stage methods and installations for the manufacture of hollow plastic products, usually bottles, preforms are obtained by continuous extrusion of a stream of thermoplastic resin, in particular polyethylene terephthalate (PET), into numerous multi-cavity molds. However, the actual manufacture of the preform depends on the method by which the preform is to be blow molded, as well as on the method of use of the finished bottle, and therefore it is necessary to properly take into account such main variables, which are listed below:
- bottle shape,
- the internal volume of the bottle,
- the type of liquid with which the bottle is supposed to be filled and which can be strongly or normally saturated or “simple”, i.e. unsaturated
- the state of the liquid, which may be hot or cold at the time of filling the bottle,
- the method of using the bottle itself, since it can be destined for destruction after use or for reuse and, therefore, reused after returning to its original state (i.e. emptying), washing, sanitizing and control, etc.

 Regarding the shape of the bottles that are reusable or, as they are commonly referred to in the art, returnable, it has been observed that the bottles are particularly suitable for reuse when the bottom or bottom of the bottle is shaped like a champagne bottle instead of " petal "form; even these two terms, in general and unmistakably, are known to those skilled in the art.

 There are two reasons for this. Firstly, the “petal” bottom is much more prone to cracking and destruction (problems caused by stress cracking) during sequential bottle treatments. Secondly, washing the bottles with a “petal” bottom for the purpose of their reuse is much more complicated, because, which is obvious, the quality of this process is significantly deteriorated in the deep recesses of the “petal” bottom due to a special shape resembling the shape of the said recesses.

 The main problem encountered in the manufacture of returnable bottles is attempts to ensure optimal distribution of the material, in particular, in the region of the support base of the bottle, and, therefore, the differential distribution of the thickness of the walls of the bottle, and therefore the differential distribution of thickness on the workpiece. In fact, given the required mechanical strength of the bottom at various pressures and temperature conditions, the bottom should have a map of a well-defined distribution of the thickness of the material.

 It is well known that while the strength of a bottle with a "petal" bottom in relation to internal pressure is ensured by its geometry, i.e. the actual shape of the mentioned bottom, and, in particular, its ribs (transverse ribs), the mentioned function of the "petal" configuration of the bottle with the shape of the bottom of the bottle for champagne can be provided only by meeting the requirements and appropriately distributed amount of material.

For example, in the case of two-liter bottles, the bottle with the shape of the bottom of the champagne bottle should have such performance that it can withstand the following processing conditions:
- three hours at 60 ^o C with 25% NaOH,
- followed by twenty-four hours in four volumes of CO ₂ at 38 ^o C.

In the event of exposure to the above test conditions, the bottom of the return bottle should provide the following performance characteristics:
1) it should not be turned out;
2) it should not be subjected to cracking due to stress or other damage of this kind;
3) the bottom should not cause loss of stability of the bottle on the support base or plane;
4) it must maintain full perpendicularity to the axis of the bottle relative to the support base or plane.

To achieve such performance, the bottle with the shape of the bottom of the bottle for champagne must have certain characteristics, which, with reference to figure 1, look as follows:
1. The maximum possible orientation of the amorphous zone 1. This is, obviously, the same zone in which the orientation tends to a minimum level, while the concentration of the material is the highest. Since the orientation of the material in this place is practically absent, mechanical strength is provided exclusively by its thickness. However, an excessively large thickness will only mean waste of material. Up to 25% of the total mass of returnable bottles is concentrated in their bottom part (zone 1 + zone 2). Thus, it is necessary to give the mentioned amorphous zone 1 the maximum possible orientation, although this may turn out to be an extremely difficult task.

 2. The maximum possible orientation of the amorphous zone 2. These are the most critical zones, since the orientation and distribution of the material are decisive conditions for the mechanical strength of the bottom.

 All bottles with the shape of the bottom of a champagne bottle, subject to internal pressure, must have a minimum thickness at the points indicated in common A and A1. At lower thicknesses, the bottle will inevitably collapse. Along with this mandatory requirement, bottles must satisfy another basic requirement, i.e. they should have a uniform uniform distribution of thickness around the circumference in accordance with zones 2. Between diametrically opposite points, at the very least, only a minimal difference in thickness may be allowed.

 For example, in the case of a two-liter return bottle with the shape of the bottom of the champagne bottle, the maximum allowable difference in thickness is from 0.2 to 0.25 mm. An important condition is to maintain this difference at a minimum, taking into account the non-perpendicularity of the bottle to a minimum.

 The degree of orientation, of course, plays a very important role, but an even more important condition is the method of distribution of the material in these zones. In both of the above methods, this distribution is largely dependent on the conditioning temperature to which the preforms are subjected to blow molding, and this treatment has two drawbacks: on the one hand, the preform wall is heated in a substantially uniform way and the result is an extremely inhomogeneous stretching of the material in blow molding process, i.e. there is a conflict situation with the requirement for differentiated distribution of the material according to a well-defined pattern in different areas of the bottle; on the other hand, a similar conditioning step in single-stage processes slows down and complicates the entire production process due to various technical and economic reasons that are known to specialists in this field of technology.

 As for the two-stage methods and the corresponding blanks, the following is characteristic of them. If necessary, due to the limitations inherent in the installation, the workpiece must be conditioned to a substantially uniform temperature, while, on the other hand, the bottom of the same workpiece must be subsequently drawn in accordance with a very differentiated scheme. It follows that in order to ensure satisfactory strength of the base or contact zone, which, as a rule, is subjected to greater drawing, it becomes necessary to use workpieces having an appropriate thickness. However, the thickness required for the said areas subjected to strong drawing is actually excessive for other areas of both the bottom and other parts of the body of the workpiece, as a result of which such a limitation in thickness ultimately results in an inefficient consumption of plastic material used in those parts of the bottle which are subjected to a degree of drawing, ranging from low to medium.

 In order to reduce such inefficient use of the material, efforts have been made to maintain the conditioning temperature within very tight limits; however, the regulation of such a technological parameter is not always easy or economically feasible.

 It follows from US Pat. No. 5,158,817 that a preformed blank for a generally rectangular container should have an oval or oblong cross section.

 However, a preformed blank of this kind is not very suitable for the manufacture of bottles having a cylindrical shape and a bottom with the shape of the bottom of a bottle for champagne.

 International application WO 90/04543 discloses a preformed preform of a particular shape. The mentioned document refers, however, to disposable bottles (disposable bottles) (see line 2 of the corresponding description); in this regard, preformed blanks of this kind are not entirely suitable for the manufacture of "returnable bottles or bottles for reuse", which is precisely the subject of this patent application.

 European Patent No. 0445465 discloses the structure of a preformed blank for returnable bottles; however, the preformed blanks presented are used for typical purposes and processing, while it is well known that due to the different heating requirements in the one-step and two-step processes, the corresponding preformed blanks must be given a corresponding and different shape.

The main disadvantages of the existing technology for the manufacture of blanks can, therefore, be presented in the following generalized form:
a) the criticality of the permissible deviations during processing, also known as the “technological window”, when the blanks for blow molding with a substantially constant wall thickness, due to the fact that the material is at the same initial temperature, must be subjected to a very different degree of drawing while maintaining a predetermined minimum thickness in the most critical area of the support base of the bottle;
b) the consequence of using blanks with a constant wall thickness is that the finished bottle has some zones of significantly greater thickness than that which would be really necessary to counter the corresponding stresses or loads, as a result of which it can be concluded that at least part the material used in these zones was practically wasted;
c) the introduction of the stage of temperature conditioning before the stage of actual blow molding in single-stage plants leads to a number of problems, the solution of which is associated with high costs and increased complexity in the design of the corresponding installation, as well as the method carried out using this installation.

Given all of the disadvantages mentioned, the main objective of the present invention, therefore, is to provide:
a) blanks for use in a single-stage method for manufacturing a container of thermoplastic resin, which is thermally stable, can be filled with both hot and saturated liquids, suitable for repeated reuse and has a bottom with the shape of the bottom of a bottle for champagne, which provides the aforementioned performance , and
b) preforms for use in a two-stage method for manufacturing a container of the type similar to that described above with reference to the one-stage method, which further reduces material consumption to a minimum level using currently available, easily and cost-effective methods of manufacturing preformed blanks of both types and eliminates the stage of conditioning the preform before the stage of blow molding only in single-stage methods.

 An additional objective of the present invention is to provide a preform having the aforementioned distinguishing features and properties, which can also be used for the manufacture of both bottles with a bottom "petal" type, and bottles that are not intended for reuse.

 Mentioned and additional objectives of the present invention will be easily and clearly understood by specialists in this field of technology after reading and understanding the following description.

 These objectives of the present invention are achievable, in fact, under the condition of a controlled combination of obtaining a certain profile and distribution of the wall thickness of the workpiece and relatively one-step processes, cooling time of the workpiece, including the exclusion of the temperature conditioning step.

A thermoplastic resin billet comprising an upper or throat section, a middle section and an end section, the lower middle section included in the middle section, on the lower side of the latter near the specified final section, adapted for stretching during blow molding, including support zones bottles and surrounding areas. The angle of inclination relative to the axis of the workpiece of the inner wall and the outer wall of said lower subsection is 5-10 ^° .

 In the indicated final section, the thickness of the preform corresponding to the lowest point is reduced to a value of 60 to 80% of the thickness of said middle section.

 Below the lower sub-section having the shape of a truncated cone, the preform regains a profile which, in fact, has a cylindrical shape, the thickness of the corresponding wall having a value of 95-100% of the thickness of the middle section.

 The preform is suitable for the manufacture of a bottle equipped with the bottom of a bottle for champagne, while the midpoints of the aforementioned sub-section, in fact, correspond to the base support zone.

 The height at which the outer diameter of the workpiece begins to decrease is slightly higher than the height at which the inner diameter of the workpiece also begins to decrease, while the height at which the gradual decrease in the outer diameter of the workpiece ends is slightly higher than the height at which the gradual decrease also ends inner diameter of the workpiece.

 The wall thickness of the preform on said lower subsection is at least 10% greater than the wall thickness of the rest of the middle portion of said preform.

A non-limiting example of the present invention is described and illustrated in detail below with reference to the accompanying drawings, in which:
FIG. 1 is a cross-sectional view of the bottom of a champagne bottle for a typical return bottle;
FIG. 2 is a cross-sectional view along the midline in the vertical plane of a workpiece according to the present invention, suitable for blow molding in a one-stage setup;
FIG. 3 is a sectional view along the midline in the vertical plane of the deformation curves of the bottom of the workpiece in accordance with the present invention;
FIG. 4 is an image of a temperature distribution map of a typical workpiece, determined when the latter exits from the injection mold;
FIG. 5 is an enlarged sectional view along the midline in the vertical plane of the blank wall shown in FIG. 2 related to a specific part of said wall;
FIG. 6 is a cross-sectional view along the midline in the vertical plane of the workpiece according to the present invention, suitable for blow molding in a two-stage setup;
FIG. 7 is a sectional view along the midline in the vertical plane of the deformation curves of the bottom of the workpiece according to the present invention during the blow molding process in a two-stage manner;
FIG. 8 is an enlarged sectional view along the midline in the vertical plane of the blank wall shown in FIG. 6, relating to a specific part of said wall.

 At the heart of the true goal, which both methods of solving, which are described in the following, are aimed at, lies a practical consideration, the essence of which is that the part of the workpiece that undergoes the maximum hood to turn it into the base of the tank is at the same time the part that, after turning into the specified base will be the most loaded, i.e., exposed to the maximum loads, and, therefore, must be able to provide the highest possible mechanical qualities, Such a basic consideration applies, in fact, to blanks of all types, i.e. both to blanks adapted for blow molding in single-stage plants, and to blanks which, in contrast, are adapted for blow molding in two-stage plants.

 The consequence of this is the determination of preforms in which the indicated part changes and is specifically “adapted” to the actual conditions of blow molding relative to the rest of the preform.

 A similar goal is achieved through a controlled combination of two main variables that affect the drawing of the workpiece, i.e. billet wall thickness and temperature during the drawing process.

 However, since the temperature at the drawing stage depends on the heat treatment and the general conditions in which the workpiece was before the mentioned stage, and such conditions, as a rule, vary significantly in the one-stage and two-stage methods, it logically follows that even the method conceived to solve this problems i.e. the optimal combination of thickness and heat treatment will be completely different in the two ways, and this is an explanation of why the continuation of the description will be divided into two different parts relating exclusively to one of the two mentioned methods.

Blank for single-stage plants and methods
The first goal to which the present invention is directed is to determine such a profile and such a map or distribution pattern for the thickness of the workpiece that would allow, after molding the specified workpiece by blowing through a one-step method, automatic distribution of plastic material in zones 1 and 2 of the bottom, which should be obtained without the aid of any intermediate methods of temperature conditioning or processing provided between the mold for injection and blowing th form.

 For a better and more complete explanation, the blank depicted in FIG. 2, is divided into three separate combined sections, i.e. the upper section or throat H, the middle section L and the final section N, on which the workpiece is compressed and closed in the form of a hemisphere. In the lower part of the middle section L of the workpiece, near the mentioned final section, the lower sub-section M is additionally allocated; the mentioned sub-section is defined as the part of the preform, which, after blow molding, becomes the supporting zone of the base of the bottle, at points A and A1, and zones 2 adjacent to such a base (Fig. 1).

 For this purpose, and with particular reference to the bottom of the champagne bottle, a preform was developed and experimentally tested which has a variable thickness distribution scheme for said lower sub-section M, which is better shown in FIG. 2 and 3.

 The basis of the method for obtaining such a profile of the distribution of thickness is the fundamental consideration, the essence of which is that the most important point for the workpiece is the presence of a certain profile of the temperature distribution, which during blow molding ensures the distribution of the material in accordance with a special necessary map or scheme thickness distribution of the bottle.

 As it was emphasized earlier, it is known that the most critical zones in this context are zones 2 located near the support base of the mentioned bottle and, first of all, points 4, which correspond to the support base itself, and for this particular reason the temperature distribution profile of the said workpiece should be such that in zones 2 of the bottom (in fact, the mentioned zones 2 are the only ring-shaped zones) a lower temperature is ensured in order to prevent excessive drawing of the material, which, m itself, reduces the thickness at the contact points 4.

 In the traditional one-step method of blow molding, the workpiece exits the injection mold with the same temperature along the entire axis of the workpiece, as a result of which there is a need for processing before blow molding to redistribute the temperature to create the optimal profile for its distribution.

 In complete contrast to the foregoing, the preform corresponding to the present invention exits directly from the injection mold with a temperature distribution profile ideal from the point of view of blow molding in accordance with the foregoing considerations.

 The main distinguishing feature of the present invention is that on the indicated lower subsection M, the outer diameter D1 and the inner diameter D2 of the workpiece are gradually reduced in accordance with the profile having the shape of a truncated cone, until they turn into D3 and D4, respectively, for re-acquisition, possibly after a short section with walls of variable thickness, of the usual hemispherical profile in the final section N.

 Such a decrease in the wall diameters primarily affects the fact that in the zones involved in the region of said narrowing, a thickness SP2 is provided that is less than the thickness SP1 of the workpiece body, which ensures that a lower temperature prevails in this part of the workpiece, since in this part of the thickness SP1 is cooling faster than adjacent areas.

 At the subsequent stage of blow molding, the lower temperature that the material has on the indicated sub-section M in the form of a truncated cone does not allow the material, despite a high degree of drawing, to undergo excessive drawing or, in any case, a degree of drawing that could be comparable to the degree of drawing to which the material of the adjacent sections is subjected, taking into account the fact that since the said material has higher temperatures, it tends to a much larger drawing, despite the fact that yanks to a lesser degree of drawing, compared with the amorphous zone adjacent to the injection point.

 It was experimentally proved that by correspondingly reducing the diameters of the workpiece in the indicated sub-section M on the specified workpiece, it is possible to obtain a differentiated cooling scheme, and in such a way as to ensure the immediate subsequent implementation of the blow molding step and to obtain a map or optimal temperature distribution scheme in order to obtain the required thicknesses the bottom of the bottle without resorting to any intermediate conditioning step.

 Therefore, on the basis of such observations and further guided by the obvious consideration that for each type of bottle a special blank is necessary for each bottle and, therefore, for the respective blank, it is possible to experimentally find the ideal thickness distribution profile to obtain the results that are the real purpose of the present invention presented in previous description.

It was especially noted the existence of a number of characteristics common to blanks of various types, which, if used independently or used in various combinations, can improve the results that can be achieved, or make it easier to obtain the necessary results. These characteristics mainly include:
- the constancy of the angle a (Fig. 2) of the inclination relative to the axis of the workpiece, the inner wall and the outer wall of said sub-section M in the form of a truncated cone, said angle being from 5 to 10 ^° ;
- a decrease in wall thickness, starting from the upper boundary SP1 down to the lower boundary SP2 of the mentioned lower sub-section M, since this unambiguously contributes to a decrease in the mass of the corresponding walls and, as a result, to further accelerate the degree of their cooling;
- a decrease in the thickness SP2 of said sub-section M by 5-10% relative to the thickness SP1 of the remaining middle portion L is also due to the fact that the walls of said sub-section M are curved inward;
- maintaining the wall thickness SP4 along the length of the lower part H of the portion P immediately below said sub-section M with a value equal to or in any case at least 5% relative to the mentioned thickness SP1, in order to obtain the maximum degree of biaxial orientation in the zone of 1 bottom;
- the wall thickness SP3 at the lowest point of the end portion N has a value of approximately 0.7 • SP1;
- the height of the mentioned sub-section M is determined experimentally in accordance with the necessary distribution of material at the bottom;
- to more effectively maintain the thickness in the contact area of the base of the bottle with the bottom of the champagne bottle, as indicated, in general, by points A and A1 in the cross section shown in FIG. 1, it was experimentally established that geometry, i.e. the shape of both the workpiece and the corresponding tool for blow molding should be such as to ensure that the specified contact zone of the base corresponds to the midpoints 4 of the mentioned sub-section M; this can be explained by the fact that such midpoints, although not to a significant degree, are subject to more intensive cooling and, therefore, are less susceptible to drawing and, consequently, to a decrease in their thickness.

 Turning to FIG. 4, which shows the temperature distribution profile determined experimentally on the workpiece according to the present invention, when the latter exits from the corresponding injection mold, it is seen that the temperature difference at points 7 and 8 is lower than in other parts of the workpiece.

 During blow molding, the indicated points 7 and 8 shown in FIG. 4 arise from zones 2 (FIG. 1) of the bottom, i.e. the material present at the indicated points 7 and 8 and corresponding to the mentioned sub-section M, in the process of blow molding is automatically flowing and is located in zones 2 of the bottom at the corresponding points A and A1 (Fig. 1) of the latter.

Further obvious benefits and advantages of the present invention have been identified in the process of systematically carried out experiments, namely:
1. The increased degree of bilateral orientation; in fact, based on the fact that on the subsection M of the preform, the diameters D3 and D4 are smaller than the diameters D1 and D2, respectively, it follows that the degree of drawing, considering ⌀ as the diameter of the support (critical zone) of the bottle, is greater, i.e. ⌀ D4> ⌀ D2 and ⌀ D3> ⌀ D1. Such an increase in the degree of orientation, in the sense of increasing the degree of drawing, is approximately 10-15% and depends on the actual size and shape of the bottle. It is generally known that an increase in the degree of orientation translates into an increase in mechanical performance and, above all, in a decrease in the cracking effect due to stress at points 4. It is well known that this effect is the worst enemy of returnable bottles.

 2. The solution to the problem of cracking of returnable bottles due to stress is of great importance for ensuring the possibility of manufacturing a workpiece in a one-step way. Regarding stress cracking, moisture is the most important element of this effect. According to the present invention, there is no risk of moisture absorption, since the preform is blown directly after being molded into an injection mold, so that there is practically no time for absorbing moisture from the environment.

 In addition, a particular advantage was established experimentally, in order to more effectively “control” the temperature in the zone subject to maximum exhaustion, that is, on the mentioned sub-section M, to ensure a decrease in wall thickness not only by simple internal narrowing of the previously mentioned angle a, but also by placing at different distances from the bottom of the workpiece the points at which the aforementioned narrowing begins and ends, as far as both the outer and inner diameters are concerned.

With reference to FIG. 5, which shows with enlargement a vertical section along the midline of the wall of the workpiece shown in FIG. 2, related to a subsection M of the said wall, the height h ₁ at which the outer diameter D1 of the preform begins to decrease should be slightly higher than the height h ₂ at which the inner diameter D2 of the preform also begins to decrease, while the height h ₃ at which ends gradually decrease in outer diameter D3 preform has a gradual reduction of the inner diameter D4, the said height differences to be somewhat above the height h _4, which also results determined according to the aforementioned angle but not bhodimym decreasing thickness of said sub-portion M.

 As a further explanation of the illustration of FIG. 5, it should be emphasized that the described heights h are related to the corresponding distance from a single initial level Z located under the workpiece and perpendicular to its X axis.

Blank for two-stage plants and methods
With reference to the blanks shown in FIG. 6, 7 and 8, which are provided for blow molding in a two-stage method, in order to facilitate a better understanding, a number of references and considerations will be used that have already been used in connection with the one-stage methods and can also be applied to typical conditions of the two-stage method.

 The blank depicted in FIG. 6, is again subdivided into three separate aligned sections, i.e., the upper section H, the middle section L and the final section N, on which the workpiece is compressed and closed in the form of a hemisphere. In the lower part of the middle section L of the workpiece, near the mentioned final section, the lower sub-section M is again highlighted; the mentioned sub-section is defined as the part of the preform, which, after blow molding, becomes the support zone of the base of the bottle, at points A and A1, and zones 2 adjacent to such a base (Fig. 7).

 For this purpose, and with particular reference to the bottom of the champagne bottle, a preform was developed and experimentally tested which has a variable thickness distribution scheme for said lower sub-section M, which is better shown in FIG. 6 and 7.

 The basis of the method for obtaining such a profile of the distribution of thickness is the fundamental consideration, the essence of which is that the most important point for the workpiece is the presence of a certain profile of the temperature distribution, which during blow molding ensures the distribution of the material in accordance with a special necessary map or scheme thickness distribution of the bottle.

 As it was emphasized earlier, it is known that the most critical zones in this context are zones 2 located near the support base of the mentioned bottle and, first of all, points 4, which correspond to the support base itself, and for this particular reason the temperature distribution profile of the said workpiece should be such that in zones 2 of the bottom (in fact, the mentioned zones 2 are the only ring-shaped zones) a lower temperature is provided in order to prevent excessive drawing of the material, which thus reducing the thickness at the contact points 4.

 In the traditional two-stage method of blow molding, work begins with the workpiece being in a cold state, which must therefore be conditioned, i.e. heat up to a temperature that is the same along the entire axis of the workpiece, as a result of which, before blow molding, there is a need for a treatment with the aim of uniformly distributing the temperature throughout the body of said workpiece.

 In complete contrast to the foregoing, the preform corresponding to the present invention comes directly from the conditioning step and therefore is available at the very beginning of the blow molding step with a temperature distribution profile ideal from the point of view of blow molding in accordance with the foregoing considerations.

 The main distinguishing feature of the present invention is that on the indicated lower subsection M, the outer diameter D1 'and the inner diameter D2' of the workpiece are gradually reduced in accordance with a truncated cone profile until they turn into D3 'and D4 'respectively, for re-acquiring the usual hemispherical profile in the final section N.

However, despite such a double narrowing, the wall thickness actually increases from the value SP1 ′ to a larger value SP2 ′, and this increase is achieved by differentiating the levels l ₁ , l ₂ at which the outer and inner diameters D1 ′ and D2 ′ respectively begin to narrow, and, similarly, by differentiating the levels of l ₃ , l ₄ , at which the outer and inner diameters D3 'and D4' respectively re-acquire their constant value in the direction of the final section N.

With reference to FIG. 8, on which, with an increase in vertical section along the midline, the wall of the workpiece shown in FIG. 6, related to the sub-section M of said wall, it should be emphasized that the various heights l ₁ , l ₂ , l ₃ , l ₄ indicated here correspond to the corresponding distance from a single initial level Z located under the workpiece and perpendicular to its X axis.

The height l ₁ at which the outer diameter D1 'of the preform begins to decrease should be lower than the height l ₂ at which the inner diameter D2' of the preform also begins to decrease, while the height l ₃ at which the gradual decrease in the outer diameter D3 'of the preform ends , should be slightly lower than the height l ₄ , at which a gradual decrease in the inner diameter D4 'also ends, and the indicated differences in height are determined in accordance with the aforementioned angle a and the necessary decrease in the thickness of the mentioned sub-section M.

 The first consequence of this special wall geometry is that on the sub-section M, on which the diameters become smaller, but the wall thickness increases, a thickness SP2 'appears, exceeding the thickness SP1' of the workpiece body. This provides an increased heat capacity of the aforementioned sub-section and, therefore, less heating in the conditioning step required by each two-stage method, due to which the said section of the preform is heated to a lesser extent than adjacent zones.

 Now it will be completely clear that such a lower temperature is precisely the result that they tried to achieve with preforms designed for blow molding in a one-step method, since the effect due to the mentioned lower temperature, i.e. a lower degree of material drawing in the zones involved is common to both methods.

 Returning again to the two-stage method, at the subsequent stage of blow molding, the lower temperature that the material has on the indicated sub-section M in the form of a truncated cone does not allow the material, despite a high degree of drawing, to undergo excessive drawing or, in any case, such a degree of drawing, which could be comparable with the degree of drawing to which the material of the adjacent areas is exposed, taking into account the fact that, since the said material has higher temperatures, it tends to zdo greater stretching, though exposed at the draw ratio as in the amorphous zone adjacent to the injection point.

 It was experimentally proved that by means of a corresponding increase in the diameters of a preform intended for blow molding by a two-stage method, it is possible to obtain a differentiated cooling scheme of the same preform on the indicated sub-section M, moreover, in such a way as to automatically obtain a map or a diagram of the optimal temperature distribution of the said preform in order to obtain the required thickness of the bottom of the bottle.

 Based on such an observation and further guided by the obvious consideration that for each bottle a special blank is necessary, for each bottle and, therefore, for the corresponding blank, it is possible to experimentally find the ideal thickness distribution profile to obtain the results that are the real purpose of the present invention presented in the previous description.

It was especially noted the existence of a number of characteristics common to blanks of various types, which, if used independently or used in various combinations, can improve the results that can be achieved, or make it easier to obtain the necessary results. These characteristics mainly include:
- the constancy of the angle of inclination a relative to the axis of the workpiece, the inner wall and the outer wall of said sub-section M in the form of a truncated cone, said angle being from 5 ^o to 10 ^o ;
- an increase in the thickness SP2 'of said sub-section M by at least 10% relative to the thickness SP1' of the remaining middle portion L due to different levels at which the narrowing of the outer and inner walls of the preform begins and ends, respectively;
- maintaining the wall thickness SP4 'along the entire length of the section P immediately below the mentioned sub-section M with a value equal to or in any case at least 5% relative to the mentioned thickness SP1', to obtain the maximum degree of biaxial orientation in the zone of 1 bottom;
- the wall thickness SP3 'at the lowest point of the end portion N has a value of approximately 0.7 • SP1';
- the height of the mentioned sub-section M is determined experimentally in accordance with the necessary distribution of material at the bottom;
- to more effectively maintain the thickness in the contact area of the base of the bottle with the bottom of the champagne bottle, as indicated by the points A and A1 in general in the cross section shown in FIG. 5, it was experimentally established that geometry, i.e. the shape of both the workpiece and the corresponding tool for blow molding should be such as to ensure that the specified contact zone of the base corresponds to the midpoints 4 of the mentioned sub-section M; this can be explained by the fact that such midpoints, although not to a significant degree, are subject to more intensive cooling and, therefore, are less susceptible to drawing and, as a result, to a decrease in their thickness.

 In the process of systematically carried out experiments, a further advantage of the present invention has been found to be that an increased degree of bilateral orientation; in fact, based on the fact that on the subsection M of the preform, the diameters D3 'and D4' are smaller than the diameters D1 'and D2', respectively, it follows that the degree of drawing, if we consider the diameter as the diameter of the support (critical zone) of the bottle, is correspondingly larger. Such an increase in the degree of orientation, in the sense of increasing the degree of drawing, is about 10-15% and depends on the actual size and shape of the bottle.

 It is generally known that an increase in the degree of orientation translates into an increase in mechanical performance and, above all, in a decrease in the effect of cracking due to stress at points 4. It is well known that this effect is the worst enemy of returnable bottles.

Claims (6)

1. A thermoplastic resin billet comprising an upper or throat section, a middle section and an end section, the lower middle section included in the middle section, on the lower side of the latter near the specified final section, adapted for stretching during blow molding, including support zones of the bottle and adjacent zones, characterized in that the angle of inclination relative to the axis of the workpiece, the inner wall and the outer wall of said lower subsection is 5-10 ^o .  2. The preform according to claim 1, characterized in that in the indicated final section, the thickness of the preform corresponding to the lowest point is reduced to a value of 60 to 80% of the thickness of said middle section.  3. The workpiece according to claim 1 or 2, characterized in that below the lower sub-section having the shape of a truncated cone, the workpiece again acquires a profile that, in fact, has a cylindrical shape, and the thickness of the corresponding wall has a value of 95-100% thickness of said middle portion.  4. The preform according to claim 3, characterized in that it is suitable for the manufacture of a bottle equipped with a bottom of a bottle for champagne, while the midpoints of the mentioned subsection, in fact, correspond to the base support zone.  5. The workpiece according to claim 3 or 4, characterized in that the height at which the outer diameter of the workpiece begins to decrease is slightly higher than the height at which the inner diameter of the workpiece also begins to decrease, while the height at which the gradual decrease in the outer the diameter of the workpiece is slightly higher than the height at which a gradual decrease in the internal diameter of the workpiece also ends.  6. The preform according to claim 5, characterized in that the wall thickness of the preform on said lower sub-section is at least 10% greater than the wall thickness of the rest of the middle portion of said preform.

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